Simplify the expression. $(a-1)(a+2)$
First distribute the ${a-1}$ onto the ${a}$ and ${2}$ $ = {a}({a-1}) + {2}({a-1})$ Then distribute the ${a}.$ $ = ({a} \times {a}) + ({a} \times {-1}) + {2}({a-1})$ $ = a^{2} - a + {2}({a-1})$ Then distribute the ${2}$ $ = a^{2} - a + ({2} \times {a}) + ({2} \times {-1})$ $ = a^{2} - a + 2a - 2$ Finally, combine the $x$ terms. $ = a^{2} + a - 2$